Architect Kazuyo Sejima wants to bring calm to cities around the globe. With a string of new projects, she's in demand

Throughout Japan's long economic funk, one street has stood firm as a stronghold of the good old days: Tokyo's hip Omotesando Avenue, where Gucci, Louis Vuitton and other name-brand boutiques have multiplied as if the bubble had never burst. The gilded strip recently got its most flamboyant address yet when Dior opened its largest shop in the world there. But the store is notable for more than the treasures for sale inside. The ultramodern glass building, which resembles a fantastically illuminated medieval castle, is also Omotesando's most striking piece of architecture. Its creator, Kazuyo Sejima, 47, recalls that Dior requested that...